(1) Field of the Invention
The present invention relates to a hydrogen-absorbing alloy electrode for use in an alkaline storage cell and its manufacturing method.
(2) Description of the Prior Art
In recent years, a metal-hydrogen alkaline storage cell employing a hydrogen-absorbing alloy for its negative electrode has attracted attention as a replacement for a nickel-cadmium storage cell. If a hydrogen-absorbing alloy of the right type having the right composition is employed, the metal-hydrogen alkaline storage cell remarkably enhances cell life and energy density. It means that developing an improved hydrogen-absorbing alloy electrode is essential for this type of storage cell.
Conventionally, a hydrogen-absorbing alloy, especially the one having a small grain size, is easy to oxidize on its surface during its pulverization, electrode production, or cell assembly. This oxidation increases contact resistance among the grains of the hydrogen-absorbing alloy, and thus lowers conductivity of the electrode for which the alloy is employed. As a result, charge-discharge efficiency, especially the one during a rapid charge discharge, is decreased. Furthermore, since the rapid charge-discharge generates hydrogen gas in the negative electrode through a side reaction, hydrogen gas partial pressure in the cell is raised. This brings about electrolyte leakage or dryout, thereby deteriorating the cell performance.
The following methods have been proposed in order to solve the above problems: a) adding a metal conductor to the hydrogen-absorbing alloy electrode as disclosed in Japanese Patent Publication Kokai No. 53-103543; and b) coating the hydrogen-absorbing alloy grains with a thin film of copper as disclosed in Japanese Patent Publication Kokai No. 50-111546.
However, these methods still involve the following problems.
a) The metal conductor, which is added in the state of a metal, not in the state of a compound, comprises large grains. Accordingly, it is extremely hard to uniformly distribute it in the electrode and its effects as a conductor are not fully obtained.
b) An additional process of coating the alloy grains with a thin film of copper complicates the procedure, which raises manufacturing cost.